1. Field of the Invention
This invention relates generally to process control circuits and, more particularly, to current sources responsive to digital signals for modulating process control devices such as valves, heaters, etc. The current sources provide analog current signals used to control temperature, flow, position, etc. as distinguished from signals used in binary applications, such as relays, lights, etc.
2. Description of the Related Art
In the related art, the use of digital to analog converter units in providing current sources has been implemented. Typically, an amplifier is used to drive the control terminal of a transistor, the transistor controlling the current through a load impedance. In certain applications, such process control applications, the value of the current through the load impedance must be controlled precisely for a digital input signal. Fluctuations in the load impedance current can be inappropriate in certain process control applications. In addition, the current source should be prevented from generating potentially destructive transient current surges in response to failure of the power supply. Digitally controlled current sources have been described in "Application Guide to CMOS Multiplying D/A Converters", copyrighted by Analog Devices Inc. The circuits of the related art do not have sufficient regulation for precision applications and can produce undesirable transients when the power supply voltage begins to fail.
A need has therefore been felt for a digitally controlled current source that provides a predetermined output current for a selected digital signal input, minimizes fluctuations in the current and prevents current surges during degradation of the circuit power supply.